1. Field of the Invention
The present invention relates to a wireless communication apparatus equipped with a plurality of antennas and a method for selecting an antenna in such a wireless communication apparatus.
2. Description of the Related Art
Wireless communication apparatuses are often used in poor communication environments. A diversity technique is developed and utilized to receive signals in such poor environments. Let it be defined here that the diversity technique is for receiving a plurality of signals carrying the same information, selecting an optimal signal from among them, or synchronizing those signals appropriately, thereby improving a receiving characteristic.
One of known configurations as a diversity technique is a method that uses a plurality of antennas for receiving the signals respectively and selecting a signal with the highest received signal level. That is, the method which switches antennas for actual use in response to the received signal level of the respective antennas (e.g., “802.11 Textbook of High Speed Wireless LAN”, edited by H. Matsue and M. Morikura, pp. 198–199, Published by IDG Japan, ISBN4-87280-490-2)
Conventional technique for switching antennas for use is a method for generating an antenna switching signal by using software, which generates the antenna switching signal by a processor executing a prescribed program and the antenna switching signal selects an antenna for actual use.
Another conventional technique is a method for generating an antenna switching signal by using a hardware circuit that comprises a counter for counting a switching timing and a series of logic circuits. An antenna for actual use is selected according to the antenna switching signal generated by the hardware circuit.
The method for generating the antenna switching signal by using software has the following problems. That is, since a processor executing the software generating the antenna switching signal usually executes other processing in parallel, it is not possible to assign the processor resource for the antenna switching signal generating process prior to other processing. Besides, a processor used for this purpose is required to be as small and inexpensive as possible. This makes it sometimes hard to perform a processing for detecting a received signal level of each antenna while sequentially selecting one from among the plurality antennas in short time. In addition, since a wireless packet arrives at a random timing, it is difficult to control the switching signal for each packet by a software processing.
The method for generating the antenna switching signal by using a hardware circuit, meanwhile, has the following problem. That is, a wireless communication apparatus detects the received signal levels by sequentially switching the plurality of antennas in relatively long time intervals during a waiting period for arrival of a packet (i.e., standby mode), hence requiring a large counter to count such a long time period and in turn making the circuit size large as a whole.